Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fixed maturities - available for sale, at market value, amortized cost	$ 15,406,572	$ 14,689,598
Equity securities, at market value, cost	0	130,287
Short-term investments, at cost	241,010	509,682
Other invested assets, at cost	$ 1,591,745	$ 1,628,753
Preferred shares, par value	$ 0.01	$ 0.01
Preferred shares, authorized	50,000,000	50,000,000
Preferred shares, issued	0	0
Preferred shares, outstanding	0	0
Common shares, par value	$ 0.01	$ 0.01
Common shares, authorized	200,000,000	200,000,000
Common shares, outstanding	69,202,000	69,044,000
Accumulated other comprehensive income (loss), net of deferred income tax expense (benefit)	$ (20,697)	$ 9,356
Treasury shares	28,551,000	28,208,000
4.868% Senior notes due 6/1/2044 [Member] | Senior Notes [Member]
Debt instrument, maturity date	Jun. 01, 2044
6.6 % Long Term Subordinated Notes [Member] | Subordinated Debt [Member]
Debt instrument, maturity date	May 01, 2067